Core and peel the apples then cut into big chunks. Place in a big bowl and drizzle lemon juice, mix well. Let it marinate for a while to soak the lemon juice. Add in flour, sugar, nutmeg, cinnamon and rum. Toss to coat well. Transfer to a 9-inch square glass pan.

Prepare topping: In a different bowl combine all dry ingredients. Add in cold butter cubes and use both hands quickly rub butter into flour mixture until loose crumbs forms. Sprinkle topping evenly on top of the nectarine, bake at preheated 375°F oven for 30 minutes. Use a fork to loose up the topping so the inner part is brown nicely, every 10 minutes until topping is all nicely brown inside out (by this point the juice from the nectarine should be thicken). Cool on the rack for 10 minutes. Serve warm, or cold with ice cream.